1. Field of the Invention
The present invention relates to a spray coating apparatus, and more particularly to a coating apparatus of the type by which articles are coated with a spray within a coating booth, for example, for use in coating lines for motor vehicle bodies.
2. Description of the Prior Art
In order to obtain coatings of satisfactory color, the coating composition to be used must be adjusted to a proper viscosity with a proper solvent. The proper solvent for the coating composition and the proper viscosity thereof differ with the internal temperature of the coating booth.
The internal temperature of the coating booth is therefore usually controlled to about 35.degree. C. in summer or to about 20.degree. C. in winter, and the amount of solvent (thinner) and the kind thereof are manually so changed that the composition has a proper viscosity as adjusted to the controlled internal temperature of the booth. Thus, the composition to be used is manually maintained at the proper viscosity.
However, the viscosity adjustment for the coating composition requires many workers. It is also difficult to exquisitely adjust the viscosity of the coating composition, and such adjustment involves problems especially when to be made in accordance with the difference in the internal booth temperature between daytime and nighttime. For example, it is cumbersome to prepare the composition with different viscosities as accurately adjusted for the day and the night. For this reason, it has been conventional practice to adjust the rate of discharge of the composition from the applicator to thereby alter the coating thickness, or to adjust the spray air pressure, in conformity with the temperature difference between the day and the night. The adjustment of the composition discharge rate, nevertheless, produces variations in the coating thickness, while it is not always easy to suitably control the means for adjusting the air pressure.
Further conventionally, the viscosity adjustment is made before the composition is delivered from a composition tank which is the composition source for the applicator, so that in the event of a color change, the previous composition present in the composition tank through the applicator must entirely be replaced by the new one and is therefore wasted in a large quantity. Further because only one kind of adjusted coating composition can be supplied to the applicator connected to a single composition source, both the undercoat and topcoat compositions are applied at the same viscosity, with the result that the applicator fails to apply the composition as adjusted to suitable viscosity of undercoat or topcoat. This becomes a limitation in giving an improved finish.